It was hitherto only possible to are the compound in question by reaction of sodium hydroxide with 2,3,4-trifluoronitrobenzene at 25.degree. C., subsequent acidification until the product precipitated and filtering off thereof according to the equation ##STR1## residual isomer contents of 2,3-difluoro-4-nitrophenol (3) occurring.
These isomer contents could be reduced by extraction of the product with non-halohydrocarbons (Ihara Chem. Ind. Co., JP 62/181,241 A2), while in the extraction with halohydrocarbons only a product of about 90 percent purity was obtained.
The addition of polar aprotic solvents proved essential for carrying out the reaction in this method because of the low solubility of 2,3,4-trifluoronitrobenzene (1) and the alkali metal salts of (2) in aqueous alkali metal hydroxide solution (Daiichi Seiyaku Co. Ltd., JP 61/246,150, 246,151, 246,171, 246,188, JP 58/135,840).
The removal of the solvent added (as a rule dimethyl sulfoxide) had to be carried out by extraction from alkaline solution by means of a further organic solvent (chloroform). After acidification of the aqueous phase removed, isolation of the product (2) was likewise carried out by extraction with chloroform (Daiichi Seiyaku Co. Ltd., JP 61/246,150, 1.11.86).